The invention relates to a cruise control road speed control device and, more particularly, to a cruise control having a speed controller and additional systems located outside the speed controller, which influence the cruise control output signal in an initial period of time of a speed control phase, with the cruise control output signal at the activation time of a speed controller phase being set to a value that is derived from the position of an associated driving-force-determining positioning element.
Cruise controls are electrical devices for controlling vehicle speed. They are activated on command from the driver, usually by means of a cruise control lever provided for this purpose. Typically, an operating lever of this kind can be used to adjust the operating modes "set and accelerate," "set and decelerate," and "resume," whereby in the first two operating modes, with only a brief actuation of the operating lever, the actual road speed is immediately set as the set road speed. When the operating lever is actuated for a longer period of time, initially the vehicle speed is changed by an acceleration control phase, and when the operating lever is released, the actual road speed prevailing at that time is set as the set road speed of the cruise control. In the "resume" mode, repeated speed control at a set road speed that is already known from a previous control phase, which is therefore already known to the cruise control, is requested, whereupon the cruise control changes the actual road speed in an acceleration control phase to this set road speed. Unless stated to the contrary, the term "acceleration" therefore also involves actual deceleration.
In the active cruise control control phases, the cruise control output signal constitutes the control signal for a control element of the motor vehicle that determines the driving force, for example by adjusting the throttle position for a motor vehicle with a gasoline engine or by adjusting the injection volume in a motor vehicle with a Diesel engine. For example, the speed controller can be a PDT.sub.1 controller. A cruise control device of this type for a commercial vehicle is described in the article "Elektronisches Gaspedal fur Nutzfahrzeuge" [Electronic Accelerator for Commercial Vehicles], Automobiltechnische Zeitschrift 95 (1993) 2, page 80, by G. Gils and A. Vokan.
Under the control influence, at the beginning of a speed control phase in cruise control operation, oscillating processes occur during the control phase which can produce a reduction in driving comfort unless additional measures are taken. Thus, when activated through the "set and accelerate" or "set and decelerate" operating modes, the cruise control knows the set road speed to be maintained at the setting time for subsequent speed control, but it has no information from the other systems as to which cruise control output signal, in other words for example what throttle position or which injection volume, is required to maintain this set road speed at the current engine load state. If the speed controller is operated, the initial speed control deviation has a value of 0 and the initial cruise control output signal corresponds to an idle position. The vehicle would therefore usually travel more slowly at first without any additional measures until a control deviation occurred and the position of the driving-force-determining positioning element, required for constant travel with the time constant of the cruise control controller-vehicle-road system, would apply. Such a vehicle-decelerating functional takeover of cruise control reduces occupant comfort.
Similar transient problems occur in the transition from the acceleration to the speed control phase, as can occur in the "set and accelerate," "set and decelerate," and "resume" operating modes. The basic difficulty here also consists in the fact that the cruise control, without additional measures, cannot determine which position of the driving-force-determining control element is required after the acceleration control phase for the constant travel to follow. In a conventional cruise control, in which the acceleration control phase is accomplished by a P control with a rampwise rising set speed value until the actual speed has reached the desired set speed, a relatively high overshoot of the actual speed then occurs which in turn leads to a subsequent phase with negative acceleration. In the "resume" mode, although the system knows the desired set speed already, so that the acceleration curve can be selected more carefully in the acceleration control phase, overshooting beyond the set speed cannot be avoided with the control unit alone, since the flat set acceleration ramp required for this results in control behavior that is too sluggish.
In order to somewhat counteract the control oscillation effects described above, most conventional cruise control systems use a pause signal that depends on the speed and which is added to the output signal for the speed controller to form the cruise control output signal. The curve of this pause signal as a function of the actual speed can be linear or curved, and is usually oriented toward the value required for constant travel on a level surface. By adding this pause signal, the speed controller then usually reaches its operating point faster. One method of operation of this type is described, for example, in U.S. Pat. No. 5,333,109. However, there, the pause signal is merely a compromise for different engine load states and, therefore, cannot ensure in most cases a completely seamless transition from accelerator operation to cruise control operation. Thus, for example, on a downhill road with a low engine load and therefore less required engine power to keep the speed constant, a jerk occurs upon engaging the cruise control as a result of an overly high cruise control starting value. This jerk is uncomfortable especially when coasting. At a higher engine load than when travelling on a level surface, for example uphill, the pause signal is insufficient to prevent the actual speed from faltering.
Systems that have been developed further, which have engine load information available, for example systems with an electronic accelerator, utilize this information by feeding it into the speed controller and adjusting its control parameters to be able to permit the speed controller to start more accurately at the actual load state. As a result of the controller recursion formulas, the influence of a control starting slate supplied in this fashion on the controller output value becomes smaller with each recursion loop, so that in fact after only a few computer cycles which are usually between 10 ms and 30 ms, the original load information no longer has a significant effect on the controller output signal and the cruise control behavior is determined almost exclusively by the speed control deviation. Despite a precise transmission of the load information set by the accelerator, the vehicle is therefore generally somewhat slower when speed control begins until speed control deviation occurs.
A system of this latter sort is disclosed in German Patent document DE 37 03 645 A1. The controller used therein operates in acceleration control phases as a pure proportional controller and in speed control phases as a PI controller with the control parameters being selectable as a function of the operating state. In particular, to change from an acceleration control phase to a speed control phase, provision is made for changing the driving-force-determining control signal abruptly at first in the direction of a smaller acceleration value and then switching from the P acceleration control to the PI speed control when the actual acceleration has dropped to zero. During this switch, the existing output value of the P controller is selected as the starting value for the integral part of the PI controller and the existing actual speed is chosen as the set speed. Swiss Patent 678 303 A5 teaches another cruise control system in which the control parameters are set at the beginning of a speed control phase as a function of the current driving state. In this system, an acceleration controller and a speed controller are provided in parallel, which are controlled by a state control. The state control always switches at appropriate times between the two controllers and sets their control parameters, whereby in particular at the switching moment to a speed control phase, the integral part of the speed controller implemented as a PI controller is set to an initial value which corresponds to the fuel supplied at the end of the previous acceleration phase. This value is further reduced by an amount that is proportional to the actual acceleration, and at the same time, the actual speed is set as the transitional set speed. As soon as the actual acceleration reaches the value of zero, the set speed is set to the actual speed which then prevails.
To improve the transient behavior of a speed controller, German Patent document DE 41 41 588 A1 describes feeding the actual speed to the set speed after a preset time function when the former has left a specified maximum range around the set speed in speed control operation. A PDT.sub.1 controller can be used in this application, for example, as a speed controller.
European Patent document EP 0 171 287 A2 describes a road speed control device with which a speed control phase can be activated by a set switch. The set speed is set to the actual speed at the activation time. A unit that can be activated by the set switch is provided in parallel with the normal speed control circuit. The unit completely controls the driving-force-determining control element for an adjustable period of time at the beginning of a speed control phase. After this period of time has elapsed, a switch is made to the normal control circuit in which the position of the control element corresponds to the difference between the actual and set speeds. The brief complete control of the driving-force-determining control element is intended to counteract the faltering of the speed at the beginning of speed control phases.
A cruise control speed control device according to this species is known from German Patent document DE 41 23 347 A1. In this device, additional electronic means ensure that the set value for a power setting element of the engine is set to a pilot value at the activation point of control. This value is derived from the position of the power setting element. Then, the normal speed control is performed without there being any further adaptation of this pilot value.
The present invention is based on the technical problem of providing a cruise control speed control device of the above-mentioned type which has an improved transient behavior for beginning speed control phases.
This problem is solved by a cruise control speed control device with a speed controller and additional systems located outside the speed controller, which influence the cruise control output signal in an initial period of time of a speed control phase, with the cruise control output signal at the activation time of a speed controller phase being set to a value that is derived from the position of an associated driving-force-determining positioning element. The additional systems incorporate a load adaptation element, which at an activation time of a speed control phase generates an initial engine load adaptation signal in such fashion that the cruise control output signal at this point in time corresponds to the current position of the associated driving-force-determining positioning element. This initial adaptation signal, up to the end of the initial period of time of the speed control phase, drops to zero with linkage to the speed controller output signal.
The load adaptation element located outside the controller first ensures that at the moment of the switch to a speed control phase, the resultant cruise control output signal will correspond to the engine load at the moment. This is so that the speed control cruise control operation that is beginning, as desired, will start with the position of the driving-force-determining control element at the switching time. In addition, the engine load adaptation element ensures that the output signal value initially prepared by it for this constant control takeover is reduced to zero during the subsequent transient phase with involvement of the output signal from the activated speed controller. Then the influence of this initial adaptation signal value can decrease during the transient phase of speed control without changing the parameters of the speed controller itself. Hence, the speed controller remains closer to the working point during the transient phase for subsequent steady-state control. At the same time, the lack of abrupt changes in the control parameters has a favorable effect on the transient behavior at the beginning of the speed control phases so that overall with these measures, an optimized transient behavior at the beginning of speed control is achieved without overshooting effects occurring to cause problems and/or undesired acceleration or deceleration effects.
There are several advantageous possibilities for reducing the engine load adaptation signal during the transient phase. Thus, for example, provision can be made to reduce the initial engine load adaptation signal value in stages by appropriate increments of the controller output signal when the controller output signal value or its change increment as a function of time has a sign which differs from the engine load adaptation signal value. Provision can be made such that in cycles in which the controller output signal and the engine load adaptation signal have the same sign, their sum, and in cycles in which they have different signs, the engine load adaptation signal reduced by the subtracted controller output signal increment, contribute to the cruise control output signal. Alternatively, it is possible to link the engine load adaptation signal and the output signal from the speed controller in the load adaptation element in such fashion that the total of the initial engine load adaptation signal value and the current controller signal value contributes to the cruise control output signal until the latter reaches a value of zero at a later point in time. By suitable controller initialization, at especially favorable points in time, the control of the previous influence of the engine load adaptation signal can be transmitted to the controller output signal and, therefore, through the controller recursion formulas to the subsequent controller behavior.
An advantageous embodiment of the present invention permits optimization of the transient behavior at the beginning of a speed control phase following a previous acceleration control phase especially in one of the "set and accelerate" or "set and decelerate" operating modes. The load adaptation element then sets the initial engine load adaptation signal to a value that corresponds to the proportional percentage of the acceleration controller at the end of the previous acceleration control phase. This in turn allows a smooth transition with very low transient effects from the acceleration control phase to the speed control phase. For additional optimization of the transient behavior, a certain minor change in the set speed value in the direction of the actual speed change at the end of the previous acceleration control phase can take place after a preset period of time. This is done in order to catch the return swing that occurs after the initial overshoot beyond the set speed that was initially set to the current actual speed, and thus smooth the transient curve subsequently.
In another preferred embodiment of the invention for such transitions from acceleration control phases to speed control phases, like those that occur in the "resume," operating mode, provision can be made at the point in time at which the ramp speed for the acceleration controller reaches the set speed, to switch from acceleration control operation to speed control operation and thereby set the dynamic, immediately effective proportional share of the acting speed controller to the proportional share of the acceleration controller at the end of the acceleration control phase. As soon as the actual speed has reached the set speed, the proportional speed of the speed controller, implemented for example as a PDT.sub.1 controller, is again set to the value provided for steady-state speed control phases.
It is understood that the above measures according to the invention can be combined with additional conventional measures for weakening the control oscillation effects, especially the provision of a pilot value that depends on speed and contributes additively to the cruise control output signal, and which can be set for an initially correct setting of the driving-force-determining control element during travel on a level surface with a typical load. An additional conventional measure that can be combined with the invention in the "resume" mode consists in changing the actual speed more slowly to the set speed by weakening the proportional shares of the acceleration and possibly also the speed controllers when the ramp speed and/or the actual speed approaches the set speed.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.